Foamable fluoropolymer composition

ABSTRACT

A foamable fluoropolymer composition containing foam cell nucleating agent is provided, wherein the fluoropolymer comprises melt-fabricable tetrafluoroethylene-/hexafluoropropylene copolymer and tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, wherein the alkyl contains 1 to 4 carbon atoms, wherein the melting temperature of said tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer is no more than 35° C. greater than the melting temperature of said tetrafluoroethylene/hexafluoropropylene copolymer, and/or wherein said tetrafluoroethylene/hexafluoropropylene copolymer and said tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer each have a melt flow rate (MFR) within the range of 1 to 40 g/10 min and the MFR of one of said copolymers is at least twice that of the other of said copolymers.

FIELD OF THE INVENTION

This invention relates to foamable fluoropolymer compositions that areespecially suitable for forming the insulation in coaxial cable to beused at very high frequencies, e.g. at least 10 GHz.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,560,829 discloses the desirability of foamedfluoropolymer insulation as the electrical insulation in cables to beused over the 3 to 18 GHz frequencies of data transmission. The use ofcertain blowing agents is disclosed together with melt-extrudablefluoropolymers having a loss tangent less than 0.0015 at 10 GHz isdisclosed. The blowing agent is dissolved in the molten fluoropolymerwithin the extruder, by virtue of the pressure maintained within theextruder. This pressure is released after the molten fluoropolymer isextruded, enabling the dissolved blowing agent to come out of solutionwithin the molten fluoropolymer to thereby form bubbles (voids) withinthe insulation extruded onto the conductor of the cable. The voids formas the molten polymer is solidifying to lock in the bubbles as cellswithin the polymer insulation, thereby forming the foamed insulation.The amount of the blowing agent present in the molten polymer isadjusted so as to remain dissolved within the extruder, but not toogreat so that the foaming of the insulation does not cause blowing outof the bubbles (rupturing) through the exposed surface of the insulationor the surface in contact with the conductor or internally within thethickness of the insulation to create large voids that deterioratesignal transmission performance of the cable. This rupturing is alimitation on the void content achievable in the extrusion foaming step.The melt strength of the fluoropolymer plays a role in this regard, i.e.the higher the melt strength of the fluoropolymer, the greater is theresistance to rupture and the greater is the void content that isachievable.

U.S. Pat. No. 4,764,538 discloses the desirability of dispersing boronnitride and certain inorganic salts into the fluoropolymer to act as anucleant for the formation of the cells (voids) within the moltenpolymer forming the foamed insulation, whereby the expansion of thedissolved blowing agent results in the cells formed within the moltenpolymer being small. The nucleating agent forms the sites for the voidsto form. This patent describes the melt draw-down extrusion foamingprocess, wherein the molten fluoropolymer containing the foam cellnucleating agent is extruded as a tube that is vacuum drawn-down intothe shape of a cone, the apex of which is the location of contact withthe wire passing through the guide tip of the extruder crosshead. Thewall thickness of the cone decreases towards the apex as thefaster-running conductor stretches (draws) the cone. The patentdiscloses that the dissolved gas comes out of solution in the moltenfluoropolymer by virtue of the sudden drop in melt pressure as themolten fluoropolymer exits the extrusion die. The evolution of thedissolved blowing agent is delayed until the molten polymer comes intocontact with the conductor so as to avoid rupture of the cone caused bythe weakening of the melt strength of the molten polymer forming thecone if voids were formed within the cone, especially in its thinningwall approaching the cone apex. The dynamic nature of the foamingprocess is revealed by Tables II and III in the '538 patent, by foamcell size varying both with the speed of the conductor (line speed) andthe length of the cone.

U.S. Pat. No. 4,877,815 discloses an additional improvement in formingfoamed insulation, i.e. of smaller cell size and higher void content,using a new class of foam cell nucleating agents, thermally stablesulfonic and phosphonic acids and salts, optionally together with boronnitride and inorganic salt.

EP 0 423 995 discloses another improvement, which is to expose thefluoropolymer to fluorine treatment to react with the unstable endgroups of the fluoropolymer to convert them to —CF₃ end groups,resulting in a reduction in dissipation factor for the fluoropolymerover the range of 100 MHz to 10 GHz. Table 1 in '995 discloses theeffect of transmission frequency on dissipation factor, namely as thefrequency increases from 1 MHz to 10 GHz, the dissipation factoradvantage of tetrafluoroethylene/perfluoro(propyl vinyl ether) copolymer(TFE/PPVE) over tetrafluoroethylene/hexafluoropropylene copolymer(TFE/HFP) reverses itself, i.e. 0.000087 vs 0.00573 to 0.0010 vs0.00084, respectively.

U.S. 2008/0283271 (U.S. Pat. No. 7,638,709) discloses improvement inreturn loss over the transmission frequency range of 800 MHz to 3 GHz byincomplete fluorination of the TFE/HFP copolymer, leaving some of theas-polymerized end groups present in the copolymer, which increases theaffinity of the foamed insulation for the conductor. The effect ofincomplete fluorination can be obtained by using a single TFE/HFPcopolymer, or a mixture of TFE/HFP copolymers, one of which isincompletely fluorinated, which saves the need for making a specialsingle fluoropolymer having the desired melt flow rate. Unfortunately,the dissipation factor of the mixture of TFE/HFP copolymers of Example 1of '271 is 0.00048, which is too high for such high transmissionfrequencies as 10 GHz.

There remains a need for foamed insulation that is both economical tomanufacture and exhibits good signal transmission properties at highfrequencies such as at least 10 GHz.

SUMMARY OF THE INVENTION

The present invention satisfies this need by the unique approach ininsulation foaming technology of forming the fluoropolymer portion ofthe foamable composition by mixing together fluoropolymers characterizedby dissimilarities and discovering that these dissimilaritiessurprisingly contribute to improved foamed insulation on the conductorof signal transmission cable.

One embodiment of the present invention is a foamable compositioncomprising melt-fabricable tetrafluoroethylene(TFE)/-hexafluoropropylene (HFP) copolymer, tetrafluoroethylene(TFE)/-perfluoro(alkyl vinyl ether) (PAVE) copolymer, wherein the alkylcontains 1 to 4 carbon atoms, and foam cell nucleating agent, themelting temperature of said TFE/PAVE copolymer being no more than 35° C.greater than the melting temperature of said TFE/HFP copolymer.

The most common TFE/HFP copolymers, commonly known as FEP, have amelting temperature of 250-260° C., and the most common TFE/PAVEcopolymers, commonly known as PFA, have a melting temperature of300-310° C. It has been found that when these copolymers, having amelting temperature difference of 45° C., form the fluoropolymer portionof the composition, wherein each of the copolymers constitute asubstantial portion, at least 25 wt % of the combined weight of thefluoropolymer portion, the extruded foamable composition formsaggregates of polymer at the exit of the extrusion die, resulting fromcopolymers phase separation and sloughing off of copolymer particles asthe extrudate exits from the die. This sloughing is different inappearance than die drool formed as a ring on the die face, encirclingthe circumference of the extrudate and periodically being pulled awayfrom the die face by the extrudate to form smooth lumps of polymer onthe surface of the foamed insulation. The die drool comes from the lowmolecular weight fraction of the fluoropolymer exuding from the surfaceof the molten fluoropolymer as it exits the extrusion die. The sloughingencountered in the workup to the present invention is characterized bydiscrete aggregates of polymer forming on the die face and being spacedaround the circumference of the extrudate. These aggregates areperiodically carried away as small flakes from the die face by theextrudate, resulting in flaws and irregularities in the foamedinsulation. This sloughing provides evidence of the immiscibility of thetwo different copolymers, which is accentuated by the high shearcondition that is characteristic of the extrusion foaming process.

This high shear arises from the necessity that the annular die openingforming the tubular shape of molten polymer extrudate has to be smallenough to maintain the molten polymer within the extruder crossheadunder sufficient pressure to keep the gas (blowing agent) in solution inthe molten polymer. This prevents the gas from forming bubbles eitherwithin the extruder or immediately after extrusion, i.e. the foaming isdelayed until the extruded tube is drawn down into contact with theconductor. Because of the small annular die opening, the draw down ratio(DDR) for foamable compositions is much less than for the extrusionformation of unfoamed fluoropolymer insulation on a conductor. Forexample, the DDR of unfoamed fluoropolymer insulation is usually in therange of 80 to 100:1, while the DDR for foamable fluoropolymercompositions is less than 30:1, more often less than 20:1. DDR forfoamed insulation is the ratio of the cross-sectional area of theannular die opening to the cross-sectional area of the insulation as itis formed in the conductor, prior to foaming of the insulation. The highshear accompanying the extrusion of foamable compositions causes thedefect of sloughing described above.

It has been discovered that surprisingly, this sloughing defect can bemitigated by using FEP and PFA that have melting temperatures closertogether, as described above.

It has also been found that surprisingly, the mixture of the differentfluoropolymers according to the present invention produces a foamedstructure characterized by small cells dispersed throughout the foamedinsulation. While the melting temperatures of the TFE/HFP copolymer andthe TFE/PAVE copolymers are closer together than would be obtained ifthe most common of these copolymers were combined, these meltingtemperatures are nevertheless different from one another, e.g. by atleast 15° C. The melting temperature difference means that the TFE/PAVEcopolymer will start solidifying before the TFE/HFP copolymersolidifies, all while bubbles are being formed within the fluoropolymerinsulation. Rather than forming regions of cell concentrationcorresponding to the lower melting TFE/HFP copolymer domains within theinsulation, the distribution of the cells is uniform throughout thefoamed insulation.

Another embodiment of the present invention is the foamable compositioncomprising melt-fabricable TFE/HFP copolymer, TFE/PAVE copolymer,wherein the alkyl contains 1 to 4 carbon atoms, and foam cell nucleatingagent, wherein the TFE/HFP copolymer and the TFE/PAVE copolymer eachhave a melt flow rate (MFR) within the range of 1 to 40 g/10 min and theMFR of one of said copolymers is at least twice that of the other ofsaid copolymers. In this embodiment, the dissimilarities are twochemically different fluoropolymers and two different melt flowcharacteristics.

Another embodiment of the present invention is the combination of theabove-described embodiments.

Still another embodiment of the present invention is the foamedinsulation made from this compositions of these embodiments and otherembodiments of the invention described herein.

DETAILED DESCRIPTION OF THE INVENTION

The essential components of the composition of the present inventionwill first be described, followed by description of their relationshipto one another.

With respect to the TFE/HFP copolymers and the TFE/PAVE copolymers thatcan be used in this invention, both copolymers are melt-fabricable andfluoroplastics. They are not fluoroelastomers. By melt-fabricable ismeant that each of these copolymers have melt flow properties andmechanical strength, such that individually each can be fabricated bysuch a common melt fabrication method as extrusion to form articleshaving good mechanical properties, manifested for example by an MIT flexlife of at least 2000 cycles (measured in accordance with ASTM D 2176 oncompression molded film 8 mils (0.21 mm) thick).

Examples of TFE/HFP copolymers that can be used in the polymer mixtureused in the present invention include the copolymers oftetrafluoroethylene (TFE) with hexafluoropropylene (HFP). Additionalcopolymerized monomers have been added to the copolymer to improve itsstrength, especially stress crack resistance, as measured by MIT flexlife. This effect is less important in the present invention because ofthe presence of the substantial amount of TFE/PAVE copolymer in thecomposition. In the composition of the present invention, the additionalmonomer is present in the TFE/HFP copolymer to improve its chemicalrelationship with the TFE/PAVE copolymer. Thus, the preferred additionalmonomer in the TFE/HFP copolymer is perfluoro(alkyl vinyl ether) (PAVE)in which the linear or branched alkyl group contains 1 to 4 carbonatoms. Preferred PAVE monomers include perfluoro(methyl vinyl ether)(PMVE), perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(propyl vinylether) (PPVE). The preferred TFE/HFP copolymers contain about 5-17 wt %HFP, and 0.2 to 4 wt % of the additional comonomer, which is preferablyPAVE such as PEVE or PPVE, the balance being TFE, to total 100 wt % forthe copolymer. The preferred HFP content of the copolymer is 9 to 12 wt%. The TFE/HFP copolymers, whether or not additional comonomer ispresent, is commonly known as FEP.

Examples of TFE/PAVE copolymers that can be used in the presentinvention include copolymers wherein the PAVE is a linear or branchedalkyl group contains 1 to 4 carbon atoms, such as perfluoro(ethyl vinylether) (PEVE) and perfluoro(propyl vinyl ether) (PPVE). These copolymersare commonly called PFA. The copolymer can be made using several PAVEmonomers, such as the TFE/perfluoro(methyl vinyl ether)/perfluoro(propylvinyl ether) (PMVE) copolymer, sometimes called MFA by the manufacturer.The TFE/PAVE copolymers have at least about 2 wt % PAVE, including whenthe PAVE is PPVE or PEVE, and will typically contain about 2-15 wt %PAVE, the remainder to total 100 wt % being TFE. When PAVE includesPMVE, the composition is about 0.5-13 wt % perfluoro(methyl vinyl ether)and about 0.5 to 3 wt % PPVE, the remainder to total 100 wt % being TFE.

The HFP or PAVE comonomer composition of the copolymers is determined byinfrared analysis on compression molded film made from the copolymer inaccordance with the procedures disclosed in U.S. Pat. No. 4,380,618 forthe particular fluoromonomers (HFP and PPVE) disclosed therein. Theanalysis procedure for other fluoromonomers are disclosed in theliterature on polymers containing such other fluoromonomers. Forexample, the infrared analysis for PEVE is disclosed in U.S. Pat. No.5,677,404. HFP content in wt % is 3.2× the HFPI, wherein HFPI (HFPindex) is the ratio of IR absorbance at 10.18 micrometers to theabsorbance at 4.25 micrometers.

The melt fabricability of each of the copolymers can also be describedin terms of melt flow rate (MFR) as measured using the Plastometer®according to ASTM D-1238-94a at the temperature which is standard forthe resin (ASTM D 2116-91a for TFE/HFP copolymer and ASTM D 3307-93 forTFE/PAVE copolymer, both specifying 372° C. as the resin melttemperature in the Plastometer®). The amount of polymer extruded fromthe Plastometer® in a measured amount of time is reported in units ofg/10 min in accordance with Table 2 of ASTM D 1238-94a. The MFR of thecopolymers used in the present invention will generally be from 1 to 40g/10 min. Melt viscosity (MV) can be calculated for MFR by therelationship 53170÷MFR in g/10 min=MV in Pa·s. Thus, the MFR range of 1to 40 g/10 min is an MV range of 5.3×10³ Pa·s to 21.3×10⁴ Pa·s. Thehigher the MFR, the lower is the MV, and the more flowable is thecopolymer in the molten state. In older literature, MV is often reportedin poises, in which case MFR can be back-calculated from the equation:MFR (g/10 min)=531700÷MV in poises.

The third component of the composition of the present invention is thefoam cell nucleating agent, which can consist of one or more compoundsthat are thermally stable under extruder processing conditions and areeffective to cause the formation of small, uniform cell sizes when thecomposition is foamed. Examples of foam cell nucleating agent includethose disclosed in U.S. Pat. No. 4,764,538, namely boron nitride incombination with certain thermally stable inorganic salts, which are inthe form of finely divided particles, or thermally stable organic acidsand salts of sulfonic acid or phosphonic acid, preferably in combinationwith boron nitride and a thermally stable inorganic salt as disclosed inU.S. Pat. No. 4,877,815. The above-mentioned acids and salts thereof arein the form of finely divided particles at room temperature, but melt attemperatures less than encountered in the extrusion foaming ofcomposition of the present invention. The preferred organic acid or salthas the formula (F(CF₂)_(n)CH₂CH₂-sulfonic or phosphonic acid or salt,wherein n is 6, 8, 10, or 12 or a mixture thereof. The sulfonic acid canbe referred to as TBSA (Telomer B sulfonic acid). Thus, a particularsalt of TBSA can be described by identity of the salt and the number ofCF₂ groups in the TBSA, e.g. KS-6 TBSA means the potassium salt of TBSAwherein 6 CF₂ groups are present in the TBSA. Another preferred organicacid and salt is the perfluoroalkanesulfonic or phosphonic acid or salt.Examples of these acids and salts are given in the following Table.

TABLE 1 ZrS-10 zirconium (+4) salt of TBSA CrS-10 chromium (+3) salt ofTBSA CeS-10 cerium (+4) salt of TBSA KS-10 potassium salt of TBSA HS-10TBSA AS-10 aluminum salt of TBSA SrS-10 strontium salt of TBSA CaS-10calcium salt of TBSA ZnS-10 zinc salt of TBSA BaS-10 barium salt of TBSALS-10 lithium salt of TBSA FS-10 iron (+3) salt of TBSA TEAS-10triethylamine salt of TBSA BS-6A barium p-(perfluoro[1,3-dimethylbfutyl]) benzene sulfonate BS-9A barium p-(perfluoro[1,3,5-trimethylhexyl]) benzene sulfonate BaS-A1(H) barium p-toluene sulfonateBaP-A barium benzene phosphonate NaP-A sodium benzene phosphonateNaS-A(II) 4,5-dihydroxy-m-benzene disulfonic acid disodium salt NaS-6sodium salt of TBSA BS-6 barium salt of TBSA BS-8 barium salt of TBSAKS-6 potassium salt of TBSA KS-8 potassium salt of TBSA KS-8C potassiumperfluorocyclohexylethane sulfonate NaS-1 sodium trifluoromethanesulfonate KS-1 potassium trifluoromethane sulfonate KS-1(H) potassiummethane sulfonate BaS-3(H) barium propane sulfonate NaTCA sodiumtrichloroacetateExamples of inorganic salts include carbonates, tetraborates,phosphates, and sulfates of such cations as lithium, sodium, potassium,and calcium. The preferred inorganic salt is calcium tetraborate.

Another component of the composition of the present invention is thecomposition within the extruder wherein blowing agent is injected intothe molten mass of the composition and becomes dissolved therein becauseof the higher pressure within the extruder. Examples of blowing agentsinclude the inert gases nitrogen, argon, neon, and carbon dioxide. Theamount of inert gas present is that which is effective to produce thevoid contents to be described hereinafter. Chemical blowing agent can beused, but the inert gas blowing agent is preferred.

The amount of each copolymer present in the composition of the presentinvention is preferably at least 25 wt %, based on the combined weightof the TFE/HFP copolymer and TFE/PAVE copolymer. On the same basis, thepreferred amount of TFE/PAVE copolymer is preferably 50 to 75 wt %,whereby the TFE/HFP copolymer content would be 25 to 50 wt %. Mostpreferably, the amount of TFE/PAVE copolymer is greater than the amountof TFE/HFP copolymer present in the composition, and up to 75 wt %,based on the combined weight of the two copolymers. The presence of thesubstantial amount of both copolymers in the composition provides bothprocessing and application benefits, but gives rise to the problem ofsloughing of the composition at the exit of the extrusion die, which isalleviated by the matching of melt temperatures as will be describedhereinafter.

The amount of foam cell nucleating agent present in the composition ispreferably that which is effective to produce the void content desired,20-65%, preferably 35 to 65% in the foamed insulation. Generally thisamount will be 0.01 to 1 wt % based on the total copolymer content ofthe composition. The proportions of the components of the foam cellnucleating agent are adjusted to obtain the cell size desired, generallyabout 50 micrometers and smaller. The small size of the cells, theirdispersion throughout the foamed insulation cell size, and the amount ofvoid content contribute to low return loss by not reflecting backtransmitted signals along the coaxial cable.

The incorporation of the foam cell nucleating agent into the copolymercomposition renders it foamable in an extrusion process in the presentof either gas injection into the polymer melt or the addition ofchemical blowing agent to the melt. The foam cell nucleating agent isincorporated into the copolymer composition either by blending with oneor both of the copolymers the form of a powder for pelletizing, so thatthe pellets include the foam cell nucleating agent mixed with the one orboth of the copolymers, or with the copolymer pellets for co-feedinginto the extruder. If both copolymers are not included in the foam cellnucleating agent-containing pellets, the pellets of the copolymer notpresent can be added to the copolymer pellets containing the foam cellnucleating agent in the desired amount as the feed to the extruder.

It is preferred that the melting temperatures of the copolymers areclose together, preferably differing by no more than 30° C., morepreferably by no more than 25° C. TFE/PAVE copolymer is the highermelting copolymer. Proximity of the melting temperatures of the TFE/HFPcopolymer and the TFE/PAVE copolymers used together in the compositionof the present invention avoids sloughing of the extrudate of thecomposition as it exits the extrusion die. The sloughing of theextrudate is a manifestation of the composition segregating as a resultof the high shear of the composition in the narrow opening through whichthe extrudate is being forced under high pressure. Unexpectedly, whenthe melting temperatures of the copolymers are closer together thanwould be obtained by selecting the most common TFE/PAVE and TFE/HFPcopolymers, this sloughing does not occur. Preferably, the TFE/HFPcopolymer has a melting temperature of 255 to 265° C. and the TFE/PAVEcopolymer has a melting temperature of 280 to 295° C. The meltingtemperatures disclosed herein are the second melting temperature (peaktemperature) of the individual copolymers, determined using DSC(differential scanning calorimeter) in accordance with the procedure inASTM D 3418, and are characterized as having a melting endotherm of atleast 3 J/g.

Change in melting temperature for the TFE/HFP copolymers and TFE/PAVEcopolymers used in the present invention primarily denotes a differencein composition of the copolymer, i.e. as the HFP and PAVE comonomercontents of the copolymers increase, the melting temperature of thecopolymer decreases. The PAVE monomer is much more expensive than boththe TFE and HFP monomers. Thus, a low melting temperature PFA representsa high cost PFA. Nevertheless, it is this TFE/PAVE copolymer, having amelting temperature of 280 to 295° C., that is preferred for use in thepresent invention.

MFR of the TFE/HFP copolymers and TFE/PAVE copolymers used in thepresent invention is primarily established by the molecular weight ofthe copolymer obtained in the polymerization process. The lower themolecular weight, the higher is the MFR. Preferably, the MFR of thecopolymers of the present invention differ from one another by a factorof at least 2×, i.e. the MFR of one of the copolymers, preferablyTFE/HFP copolymer, is at least 2× that of the MFR of the othercopolymer, preferably TFE/PAVE copolymer. More preferably, the TFE/PAVEcopolymer has the lower MFR, e.g. having an MFR of no greater than 15g/10 min, preferably no greater than 10 g/10 min, and the TFE/HFPcopolymer has an MFR of at least 20 g/10 min, preferably at least 24g/10 min, and most preferably at least 28 g/10 min.

Surprisingly, and notwithstanding that the TFE/HFP and TFE/PAVEcopolymers are chemically different from one another, the high MFRTFE/HFP copolymer increases both the melt flowability of the compositionin the extrusion foaming process and the foamability of the compositionto form the foamed insulation. TFE/PAVE copolymer is well known to havea substantially higher melt strength than TFE/HFP copolymer at the sameMFR. This melt strength difference is accentuated by the preferredTFE/PAVE copolymer having the low MFR and the preferred TFE/HFPcopolymer having the high MFR. The higher the melt strength, the moreresistant is the molten polymer to foaming. Nevertheless, thecomposition of the present invention acts more like the TFE/HFPcopolymer by itself than the TFE/PAVE copolymer by itself in both easeof extrudability and extent of foaming. These benefits are measured ashigher than expected line speeds in the extrusion foaming process andhigher than expected void contents in the foamed insulation.Notwithstanding the difference in chemical identities of the twocopolymers, with the TFE/PAVE copolymer having the higher meltingtemperature and preferably having the lower MFR, the cell structureformed within the foamed insulation is characterized by small cell sizesand uniform distribution of the cells throughout the insulation. Thecells (voids) do not form clusters within the foamed insulation as wouldbe expected from the presence of the more easily foamable lower melting,higher MFR TFE/HFP copolymer.

The copolymers used in the composition of the present invention alsopreferably have a compositional relationship. In this regard, it ispreferred that these copolymers have a common comonomer in addition tothe TFE. Since TFE/PAVE already has PAVE comonomer, it is thereforepreferred that PAVE comonomer also be present in the TFE/HFP copolymer.Most preferably the PAVE comonomer is the same in each copolymer. It hasbeen found that for this compositional relationship to be effective, thecommon comonomer present in each copolymer has to be present in aneffective amount. The effect of the common comonomer is to alleviate thesloughing that can occur when the composition is extruded in theextrusion foaming process. In the most preferred composition, the PAVEcontent of the TFE/PAVE copolymer is sufficiently high that it iseffective to provide the needed composition matching between thecopolymers to prevent extrusion sloughing. The TFE/PAVE copolymershaving a melting temperature of about 305° C. generally have 4 wt % andless of the PAVE comonomer. As the PAVE content is increased, themelting temperature decreases, and this PAVE increase in the TFE/PAVEcopolymer is the preferred way for providing the low melting temperatureTFE/PAVE copolymer used in the present invention to be within 35° C. orless than the melting temperature of the TFE/HFP copolymer. Preferably,the TFE/PAVE copolymer used in the present invention contains at least 6wt % PAVE.

For optimum signal transmission properties from cable wherein thecomposition of the present invention forms the foamed insulation aroundthe central conductor of the cable, as in coaxial cable, it is preferredthat both copolymers have stable end groups, especially the —CF₃ endgroups. Such end groups can be obtained as a result of thecopolymerization process, wherein the initiator contains the —CF₃ group.More common, however, is the use of the aqueous dispersioncopolymerization process, wherein the end groups resulting from suchpolymerization include —COF, —CONH₂, —CH₂OH, and —COOH and these endgroups can be converted to —CF₃ end groups by fluorine treatment of theas-polymerized copolymer, such as disclosed in U.S. Pat. Nos. 4,743,658and 6,838,545. This fluorination treatment is carried out on thecopolymer prior to blending with the foam cell nucleating agent.According to one embodiment of the present invention, no more than 20unstable end groups/10⁶ carbon atoms are present in both copolymers,preferably no more than 6 such end groups/10⁶ carbon atoms.

According to another embodiment of the present invention, either or bothof the copolymers contains a moderate amount of the as-polymerizedunstable end groups described above to increase the affinity of thefoamed insulation for the conductor so that the composition exhibits astrip force, preferably at least 3 lbf (13.3 N), to provide good andconsistent adhesion between the foamed insulation and the conductor.Strip force is the force necessary to break the adhesive bond betweenthe foamed insulation and the conductor and is determined as disclosedhereinafter. Each of the copolymers when made by aqueous dispersioncopolymerization has greater than 400 end groups/10⁶ carbon atoms. Themoderate amount of unstable end groups is preferably 30 to 120 unstableend groups/10⁶ carbon atoms, provided by one or both of the copolymers.This moderate amount of unstable end groups is obtained by incompletefluorination of the as-polymerized copolymer, as compared tofluorination that provides no more than 20 unstable end groups/10⁶carbon atoms. It is preferred that only one of the copolymers isincompletely fluorinated, whereby one of the copolymers will contain 30to 120 unstable end groups/10⁶ carbon atoms, while the other copolymerwill contain no more than 20 unstable end groups/10⁶ carbon atoms,preferably no more than 6 such end groups/10⁶ carbon atoms. In eachcase, the remaining number of end groups are —CF₃.

The process of the present invention comprises forming a compositioncomprising melt-fabricable tetrafluoroethylene/hexafluoropropylenecopolymer, tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer,wherein the alkyl contains 1 to 4 carbon atoms, foam cell nucleatingagent, and blowing agent melt draw-down extruding said compositionthrough an extrusion die onto a conductor to form insulation on saidconductor, said melt draw-down extruding forming a cone of saidcomposition in the molten state extending from said die to the formationof said insulation on said conductor, wherein the melting temperature ofsaid tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer being nomore than 35° C. greater than the melting temperature of saidtetrafluoroethylene/hexafluoropropylene copolymer and being effective toprevent sloughing from the extrudate (cone), and/or wherein saidtetrafluoroethylene/hexafluoropropylene copolymer and saidtetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer each have amelt flow rate (MFR) within the range of 1 to 40 g/10 min and the MFR ofone of said copolymers is at least twice that of the other of saidcopolymers, said blowing agent causing said molten composition to foamon said conductor. Another embodiment of the present invention is thefoamed insulation made by this process.

Preferably, the melt draw-down extrusion is carried out at a draw downratio (DDR) of no greater than 30:1 and the axial length of said cone isat least 1 in (2.54 cm). Preferably, the insulation will have a voidcontent of 35 to 65%, more preferably, 45 to 60%. It is also preferredthat the insulation is at least 20 mils (0.5 mm) thick. Thesepreferences define some of the unique extrusion characteristics of thecomposition of the present invention. The cone length, preferably atleast 2 in (5.08 cm) that can be achieved with the composition of thepresent invention promotes a higher level of foaming, while providingintimacy of the foamed insulation with the conductor, resulting inreduced return loss. It is unexpected that such long cone length can beachieved in view of the differences in chemical composition, MFR, orboth, with respect to the two copolymers present in the composition. Inother words, the long cone length provides improvement notwithstandingthe disparate nature of the copolymers of the composition. The effect ofthis disparate nature is evidenced when the insulation wall thickness istoo small. At 50% void content, the 20 mil (0.5 mm) thickness of thefoamed insulation corresponds to 12 mils if the insulation wereunfoamed. As the wall thickness decreases from 12 mils (0.3 mm)(unfoamed basis), the molten copolymer cone tends to rupture, creatingopenings in the foamed insulation. The low draw-down ratio, preferablyno greater than 20:1, tends to provide thin walled cones of moltencopolymer. Rupture of the cone can be avoided by slowing down the linespeed, resulting in a loss of productivity. It is therefore preferredthat the insulation wall thickness, on an unfoamed basis, which is thewall thickness of the insulation initially formed on the conductor justprior to foaming to expand the insulation wall thickness, is at least 12mils (0.3 mm).

While line speed is a concern in the production of foamed insulation bythe process of the present invention, the line speed is low as comparedto line speeds achievable for melt draw down extrusion coating ofconductor to form thin-walled unfoamed (solid) primary insulation ofTFE/HFP copolymer, e.g. 10 mil (0.25 mm) thick, depending on electricalrequirements of the application for the insulated wire. As insulationuniformity and conductor concentricity requirement increase, line speedhas to be decreased to satisfy the more rigorous signal transmissionrequirements. Nevertheless, the line speed for such solid copolymerinsulation can be at least 1000 ft/min (305 m/min) to satisfy the rangeof signal transmission requirements. In contrast, the line speed formelt draw-down extrusion to form thick foamed insulation is generallyless than 300 ft/min (91.4 m/min). For example, heretofore, theformation of 40 mil (1.0 mm) thick foamed insulation having a voidcontent of 50%, could be carried out at a line speed of no more thanabout 150 ft/min (45.7 m/min).

The present invention is especially useful as the foamed insulation oncoaxial cable, wherein the thickness of the foamed insulation is atleast 20 mils thick, preferably at least 30 mils thick.

Cable using the foamed insulation made from the composition of thepresent invention exhibits very favorable signal transmissionproperties. The dissipation factor for the composition of the presentinvention is preferably no greater than 0.00032, preferably no greaterthan 0.00030 at 10 GHz. The foamed insulation also exhibits veryfavorable properties at lower frequencies. For example, the return lossover the range of 800 MHz to 3 GHz is no greater than −26 dB. For returnloss of the insulation, the higher the negative number, the better(smaller) is the return loss. Just the opposite is true in themeasurement of attenuation, measured in dB/100 ft (30.5 m), of thecable. The smaller the negative number, the better (smaller) is theattenuation. For 40 mil (1 mm) thick insulation having a void content of45%, the attenuation improvement for insulation made from composition ofthe present invention steadily increases over the attenuation of foamedinsulation of the same thickness and void content made from a blend ofTFE/HFP copolymers in accordance with US 2008/-0283271 (see Example 3).Cables made from foamed insulation of the composition of the presentinvention preferably have an attenuation that is no greater than −22dB/100 ft (30.5 m) at 3 GHz.

EXAMPLES

Two TFE/PAVE copolymers are used in the Examples, as follows:

PFA A is a copolymer of TFE and 3.7 wt % PPVE having an MFR of 5.4 g/10min, which has been subjected to fluorine treatment so as to have —CF3end groups replacing unstable end groups, with no more than a total of 6such end groups, as described above, remaining in the copolymer. Thiscopolymer has a melting temperature of 305° C.

PFA B is a copolymer of TFE and 7.3 wt % PEVE having an MFR of 6.6 g/10min, which has been subjected to fluorine treatment so as to have —CF3end groups replacing unstable end groups, with no more than a total of 6such end groups, as described above, remaining in the copolymer. Thiscopolymer has a melting temperature of 288° C.

The TFE/HFP copolymer used in these Examples contains 10 to 11 wt % HFPand 1-1.5 wt % PEVE, the remainder being TFE. This FEP has an MFR 30g/10 min and has about 50 wire affinity end groups per 10⁶ carbon atoms,these wire affinity end groups, primarily —COOH, arising from thepolymerization process. The remaining end groups are the stable —CF₃ endgroup obtained by fluorination of the FEP. The extruder fluorinationprocedure of Example 2 of U.S. Pat. No. 6,838,545 (Chapman) is usedexcept that the fluorine concentration is reduced from 2500 ppm in the'545 Example to 900 ppm.

The foam cell nucleating agent is a mixture of 91.1 wt % boron nitride,2.5 wt % calcium tetraborate and 6.4 wt % of the barium salt of telomerB sulfonic acid, to total 100% of the combination of these ingredients,as disclosed in U.S. Pat. No. 4,877,815 (Buckmaster et al.). This agentis provided as a 4 wt % concentrate in the FEP described in thepreceding paragraph, based on the total weight of the concentrate.

To form a foamable PFA/FEP composition, extruded pellets of the foamcell nucleating agent concentrate are dry blended with pellets of thePFA and FEP and then subjected to the extrusion wire coating/foamingprocess.

Return loss is determined on a 1000 ft (305 m) length of coaxial cableby measuring signal loss in both directions along the cable andaveraging the two measurements. The signal loss is measured at 1601frequencies uniformly spaced apart in a sequential frequency sweep from0 MHz to 4.5 GHz and the return losses from this sweep are averaged toobtain the average return loss over this range of frequencies. AnAgilent Technologies Network Analyzer can be used to make thesemeasurements and provide readout of the average return loss. The sameAnalyzer is used to measure attenuation on 100 ft (30.5 m) lengths ofcable.

Strip force is the force necessary to break the adhesive bond betweenthe foamed insulation of the coaxial cable and the wire conductor and isdetermined on a length of coaxial cable consisting of 3 in (7.6 cm) ofthe coaxial cable and 1 in (2.5 cm) of copper conductor with the foamedinsulation and overlying outer conductor stripped away. The wireconductor is copper since that is the most common wire conductormaterial. This length of coaxial cable is placed in a slot within astationary metal plate, the slot being wide enough to accommodate thecentral conductor pointing downwardly, but not to permit passage of theportion of the coaxial cable containing the foamed insulation and outerconductor through the slot. The downwardly extending copper conductor isgripped by a jaw of an INSTRON® tensile testing machine and the jaw ismoved away from the slot at a rate of 2.5 cm/min. The strip force is theforce causing the foamed insulation to breakaway from the copperconductor so that the conductor can then be pulled from the foamedinsulation. This test is carried out at ambient temperature (15° C. to20° C.), and the temperature of the wire conductor at which the foamablecomposition is applied to the conductor is no greater than about 200° F.(93° C.).

Void content of the foamed insulation is calculated from the equation:Void Content (%)=100×(1−d _((foamed)) /d _((unfoamed))).

The density of the foamed insulation is determined by cutting a lengthof insulated conductor, removing the insulation, calculating the volumein cubic centimeters of the insulation and dividing that value into theweight in grams of the insulation. The density is the average ofmeasurements of at least 5 samples, each being ˜30 cm in length. Thedensity of the unfoamed insulation is 2.15 g/cm³.

Dissipation factor is measured on compression molded plaques inaccordance with ASTM D 2520 as disclosed in EP 0 423 995.

Example 1

A dry blend of 56 parts by weight of PFA B and 44 parts by weight of theFEP, together with foam cell nucleating agent concentrate is formedwherein the wt % of the nucleating agent is 0.30 wt %, based on thetotal weight of the composition. The MFR of the blend is 11.9 g/10 min.The extrusion foaming conditions are conventional. The extruder isinjected with nitrogen gas at high pressure. The draw down ratio (e.g.DDR, in a tubular die, is defined as the ratio of the cross-sectionalarea of the annular die opening to the cross-sectional area of thefinished insulation) of the extruded fluoropolymer composition is about7 and the temperature of the copper conductor is ambient temperature,i.e. no pre-heat is applied. The extrusion conditions are such that thefoaming is delayed until the extruded polymer is in contact with thecopper conductor. The length of the molten copolymer cone is 2 in. (5.08cm). The line speed of the extrusion foaming process is 145 ft/min (44.2m/min). There is no formation of flakes on the die face during theextrusion foaming run.

The foam insulated wire is then formed into a coaxial cable byconventional procedure, including the braiding of strips of conductivemetal over the foamed insulation to form the outer conductor and theapplication of a polymer jacket over the outer conductor. The dimensionsof the coaxial cable are 0.0228 in (0.58 mm) diameter central conductorand outer foam diameter of 0.1020 in (2.6 mm), whereby the thickness ofthe foamed insulation is about 0.040 in (1.0 mm). The void content ofthe foamed insulation is 47%. The composition of the foamed insulationexhibits a dissipation factor of 0.0003 at 10 GHz.

This coaxial cable exhibits a return loss of −30 dB at 1 GHz, and thestrip force required to break the adhesion between the foamed insulationand the central conductor is 6 lbf (25.5 N). Similar results areobtained when the foam cell nucleating agent is mixed directly with thePFA B/FEP composition to be extrusion foamed, rather than using apolymer concentrate of the foam cell nucleating agent.

The composition exhibits a dissipation factor at 10 GHz of 0.0003 andthe cable made fro the foamed composition as its insulation exhibits anattenuation at 3 GHz of −22.5 dB/100 ft (30.5 m). The foamed insulationexhibits a capacitance of 16.8 pf/ft, (55.1 pf/m) indicating a highlyfoamed structure.

Example 2

In this Example, a different cable is made, the conductor having adiameter of 0.0183 in (0.46 mm) and an outer diameter for the foamedinsulation of 0.074 in (1.88 mm), the thickness of the foamed insulationbeing 0.0279 in (0.71 mm). Three foamable compositions are prepared forseparate extrusion foaming to form this insulation, as follows:

Composition 1: The same as in Example 1.

Composition 2: The same as in Example 1 except that PFA B is replaced byPFA A.

Composition 3: A melt blend of the FEP copolymers of Example 1 of US2008/0283271 (U.S. Pat. No. 7,638,709), except that FEP A (MFR 7 g/10min)) and FEP B (MFR 30 g/10 min) proportions are 60 wt % and 40 wt %,respectively to match the MFR of Composition 1, resulting in an MFR forComposition 3 of 12.2 g/10 min.

Compositions 1 and 2 are capable of being melt draw-down extrusionfoamed at a line speed of 360 ft/min (109.7 m/min), while forcomposition 3, the line speed can be no more than 310 ft/min (94.5m/min).

The extrusion foaming of Composition 1 produces a high void content of49.5%, and cable attenuation at 1 GHz of −18.8 dB/100 ft (30.5 m).

The extrusion foaming of Composition 2 is accompanied by sloughing offof polymer to form aggregates on the die face spaced from each other andaround the extrudate, which are periodically carried away by theextrudate to form rough particles adhered to and extending from theexposed surface of the foamed insulation, whereby the resultant foamedinsulation is unacceptable. The void content for this foamed insulationis only 44.5% and the attenuation of the cable is −19.2 dB/100 ft (30.5m).

The extrusion foaming of Composition 3 produces a void content of 46.2%and the cable attenuation is −19.6 dB/100 ft (30.5 m). The extrusionfoaming of Composition 1 can be carried out at a greater line speed thanwhen Composition 3 is used, and produces better signal transmission(attenuation) results.

The extrusion foaming of Compositions 1 and 3 is not accompanied bysloughing as occurs in the extrusion foaming of Composition 2.

Example 3

In this Example, signal transmission results are compared for a a cablewherein the foamed insulation has a void content of 45% and thethickness of the foamed insulation is 0.040 in (1.0 mm). The comparisonis between the foamed insulation made from the composition of Example 1of the present specification and the foamed insulation made from thecomposition of Example 1 of US 2008/0283271 (U.S. Pat. No. 7,638,709),except that the concentration of the foam cell nucleant is 0.3 wt %. Theattenuation of the cable of Example 1 of the present specification isbetter than that for the all FEP foamed insulation over the entiremeasured range of 800 MHz to 4.5 GHz. Thus, at 1000 MHz, 2 GHz, 3 GHz,and 4 GHz, the attenuation improvement is 1 dB/100 ft (30.5 m), 2 dB/100ft (30.5 m), 3 dB/100 ft (30.5 m), and 4 dB/100 ft, respectively, forthe present invention. At 3 GHz, for example, the attenuation of thecable made from insulation of the present invention is −21.5 db/100 ft(30.5 m) as compared to −23.5 dB/100 ft (30.5 m) for the TFE/HFPcopolymer blend composition of '271 Example 1.

1. A foamable composition comprising melt-fabricabletetrafluoroethylene/hexafluoropropylene copolymer,tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, wherein thealkyl contains 1 to 4 carbon atoms, and foam cell nucleating agent, themelting temperature of said tetrafluoroethylene/perfluoro(alkyl vinylether) copolymer being no more than 35° C. greater than the meltingtemperature of said tetrafluoroethylene/hexafluoropropylene copolymer,wherein said tetrafluoroethylene/hexafluoropropylene copolymer contains0.2 to 4 wt % perfluoro(alkyl vinyl ether), wherein said alkyl contains1 to 4 carbon atoms and perfluoro(alkyl vinyl ether) of saidtetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer is present inan effective amount to prevent sloughing of said composition at theexterior of an extrusion die in the extrusion foaming of saidcomposition.
 2. The foamable composition of claim 1, wherein said amountis at least 6 wt % based on the combined weight of saidtetrafluoroethylene and said perfluoro(alkyl vinyl ether).
 3. A foamablecomposition comprising melt-fabricabletetrafluoroethylene/hexafluoropropylene copolymer,tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, wherein thealkyl contains 1 to 4 carbon atoms, and foam cell nucleating agent, themelting temperature of said tetrafluoroethylene/perfluoro(alkyl vinylether) copolymer being no more than 35′C greater than the meltingtemperature of said tetrafluoroethylene/hexafluoropropylene copolymer,wherein the melt flow rate (MFR) of saidtetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer is within therange of 1 to 15 g/10 min and the MFR of saidtetrafluoroethylene/hexafluoropropylene copolymer is within the range of20 to 40 g/10 min and the MFR of saidtetrafluoroethylene/hexafluoropropylene copolymer is at least twice thatof said tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer.
 4. Afoamable composition comprising melt-fabricabletetrafluoroethylene/hexafluoropropylene copolymer,tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, wherein thealkyl contains 1 to 4 carbon atoms, and foam cell nucleating agent,wherein said tetrafluoroethylene/hexafluoropropylene copolymer and saidtetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer each have amelt flow rate (MFR) within the range of 1 to 40 g/10 min and the MFR ofone of said copolymers is at least that of the other of said copolymers,wherein said tetrafluoroethylene/hexafluoropropylene copolymer alsocontains 0.2 to 4 wt % perfluoro(alkyl vinyl ether) wherein the alkylcontains 1 to 4 carbon atoms.